5 quark baryon created

A FIVE-QUARK STATE HAS BEEN DISCOVERED, first reported by a group of
physicists working at the SPring-8 physics lab in Japan. All
confirmed particles known previously have been either
combinations of three quarks (baryons, such as protons or neutrons)
or two quarks (mesons such as pions or kaons). Although not
forbidden by the standard model of particle physics, other
configurations of quarks had not been found until now. The
"pentaquark" particle, with a mass just above 1.5 GeV, was
discovered in the following way. At the SPring-8 facility a laser
beam is scattered from a beam of 8-GeV electrons circulating in a
synchrotron racetrack. These scattered photons constitute a beam of
powerful gamma rays which were scattered from a fixed target
consisting of carbon-12 atoms. The reaction being sought was one in
which a gamma and a neutron inside a carbon nucleus collided,
leaving a neutron, a K+ meson, and a K- meson in the final state.
Efficient detectors downstream of the collision area looked for the
evidence of the existence of various combinations of particles,
including a
short-lived state in which the K+ and the neutron had coalesced
(drawing will be posted soon at www.aip.org/mgr/png[/URL] ). In this
case the amalgamated particle, or resonance, would have consisted of
the three quarks from the neutron (two "down" quarks and one "up"
quark) and the two quarks from the K+ (an up quark and a strange
antiquark). The evidence for this collection of five quarks would
be an excess of events (a peak) on a plot of "missing" masses
deduced from K- particles seen in the experiment. The
Laser-Electron Photon Facility (LEPS) at the SPring-8 machine
([url]http://www.rcnp.osaka-u.ac.jp/Divisions/np1-b/index.html[/url] ) is
reporting exactly this sort of excess at a mass of 1540 MeV with an
uncertainty of 10 MeV. The statistical certainty that this peak is
not just a fluctuation in the natural number of background events,
and that the excess number of events is indicative of a real
particle, is quoted as being 4.6 standard deviations above the
background. This, according to most particle physicists, is highly
suggestive of discovery. (Nakano et al., Physical Review Letters, 4
July 2003; contact Takashi Nakano, [email]nakano@rcnp.osaka-u.ac.jp[/email],)
Confirmation of this discovery comes quickly. A team of physicists
in the US, led by Ken Hicks of Ohio University (hicks@ohio.edu,
740-593-1981) working in the CLAS collaboration at the Thomas
Jefferson National Accelerator Facility, has also found evidence for
the pentaquark. A photon beam (each photon being created by
smashing the Jefferson Lab electron beam into a target and then
measuring the energy of the scattered electron in order to determine
the energy of the outgoing gamma) was directed onto a nuclear
target. The photon collides with a deuteron target and the
neutron-kaon (nK+) final state is studied in the CLAS detector
([url]http://www.jlab.org/Hall-B/[/url] ). The Jefferson Lab result was
announced at the Conference on the Intersections of Nuclear and
Particle Physics ([PLAIN]http://www.cipanp2003.bnl.gov [Broken] ) held on May 19-24,
2003, at New York City. Stepan Stepanyan (stepanya@jlab.org,
757-269-7196) reported at this meeting that the mass measured for
the pentaquark, 1.543 GeV (with an uncertainty of 5 MeV), is very
close to the LEPS value. The statistical basis of the CLAS
measurement is an impressive 5.4 standard deviations. (This result
is about to be submitted to Physical Review Letters.) These
results, together with the previous results from SPring-8, now
provide firmer evidence for the existence of the pentaquark. The
HERMES experiment at the DESY lab in Germany is also pursuing the
pentaquark particle.
The discovery of a 5-quark state should be of compelling interest to
particle physicists, and this might be only the first of a family of
such states. Not only that but a new classification of matter, like
a new limb in the family tree of strongly interacting particles:
first there were baryons and mesons, now there are also
pentaquarks. According to Ken Hicks, a member of both the SPring-8
and Jefferson Lab experiments, this pentaquark can be considered as
a baryon. Unlike all other known baryons, though, the pentaquark
would have a strangeness value of S=+1, meaning that the baryon
contains an anti-strange quark. Past searches for this state have
all been inconclusive. Hicks attributes the new discovery to better
beams, more efficient detectors, and more potent computing analysis
power. (Additional website:http://www.phy.ohiou.edu/~hicks/thplus.htm )